Riparian Condition Assessment for the Pawnee River Fort Larned National Historic Site, Kansas
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National Park Service U.S. Department of the Interior Natural Resource Stewardship and Science Riparian Condition Assessment for the Pawnee River Fort Larned National Historic Site, Kansas Natural Resource Report NPS/NRSS/WRD/NRR—2015/912 ON THE COVER Army bridge built in 1868 across the Pawnee River at Fort Larned Source: Fort Larned National Historic Site archives Riparian Condition Assessment for the Pawnee River Fort Larned National Historic Site, Kansas Natural Resource Report NPS/NRSS/WRD/NRR—2015/912 Michael Martin National Park Service Water Resources Division 1201 Oakridge Drive, Suite 250 Fort Collins, Colorado 80525 Joel Wagner National Park Service Water Resources Division P.O. Box 25287 Denver, Colorado 80225 February 2015 U.S. Department of the Interior National Park Service Natural Resource Stewardship and Science Fort Collins, Colorado The National Park Service, Natural Resource Stewardship and Science office in Fort Collins, Colorado, publishes a range of reports that address natural resource topics. These reports are of interest and applicability to a broad audience in the National Park Service and others in natural resource management, including scientists, conservation and environmental constituencies, and the public. The Natural Resource Report Series is used to disseminate comprehensive information and analysis about natural resources and related topics concerning lands managed by the National Park Service. The series supports the advancement of science, informed decision-making, and the achievement of the National Park Service mission. The series also provides a forum for presenting more lengthy results that may not be accepted by publications with page limitations. All manuscripts in the series receive the appropriate level of peer review to ensure that the information is scientifically credible, technically accurate, appropriately written for the intended audience, and designed and published in a professional manner. This report received formal peer review by subject-matter experts who were not directly involved in the collection, analysis, or reporting of the data, and whose background and expertise put them on par technically and scientifically with the authors of the information. Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily reflect views and policies of the National Park Service, U.S. Department of the Interior. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. Government. This report is available in digital format from the Natural Resource Publications Management website (http://www.nature.nps.gov/publications/nrpm/). To receive this report in a format optimized for screen readers, please email [email protected]. Please cite this publication as: Martin, M. and J. Wagner. 2015. Riparian condition assessment for the Pawnee River, Fort Larned National Historic Site, Kansas. Natural Resource Report NPS/NRSS/WRD/NRR—2015/912. National Park Service, Fort Collins, Colorado. NPS 425/127821, February 2015 ii Contents Page Figures ................................................................................................................................................... iv Executive Summary ............................................................................................................................... v Acknowledgments ................................................................................................................................. vi Introduction and Setting ......................................................................................................................... 1 Pawnee River Channel Characteristics .................................................................................................. 3 Recent Geomorphic History of the Pawnee River ................................................................................. 9 Hydrology of the Pawnee River ........................................................................................................... 10 Groundwater and Riparian Resources .................................................................................................. 12 Functional Condition of Riparian Systems .......................................................................................... 14 Assessment Results and Discussion ..................................................................................................... 16 Hydrology ..................................................................................................................................... 16 Vegetation .................................................................................................................................... 17 Erosion/Deposition ....................................................................................................................... 18 Conclusions/Recommendations ........................................................................................................... 20 References ............................................................................................................................................ 22 Appendix: PFC Checklist for Riparian Condition Assessment ........................................................... 23 iii Figures Page Figure 1. Location of Fort Larned along the Santa Fe Trail. ................................................................ 1 Figure 2. A typical view of the Pawnee River channel at FOLS (NPS, 2013). ................................... 3 Figure 3. Topographic overview of the Pawnee River Valley and some of the adjacent uplands.. ................................................................................................................................................. 5 Figure 4. Google Earth image of the Pawnee River at FOLS. .............................................................. 6 Figure 5. Oblique aerial Google Earth image of the check dam that spans the channel of the Pawnee River about 500 feet downstream of the park boundary .......................................................... 7 Figure 6. Concrete check dam located just downstream from FOLS. .................................................. 8 Figure 7. Photo of first army bridge built in 1868 across the Pawnee River at FOLS. ......................... 9 Figure 8. Annual peak flow record from USGS gage #07141200, Pawnee River at Rozel, KS. ....... 10 iv Executive Summary The National Park Service (NPS) is preparing a Natural Resource Condition Assessment for Fort Larned National Historic Site. To inform this assessment, the NPS Water Resources Division was asked to evaluate the functional condition of the Pawnee River riparian area within and adjacent to the park. We performed our assessment during a June 2013 site visit. Using methods described in “A User Guide to Assessing the Proper Functioning Condition and the Supporting Science for Lotic Areas” (U.S. Department of the Interior, 1998), we determined that the entire reach of the Pawnee River at Fort Larned is in a “nonfunctional” condition (lowest of three possible ratings). This rating is given to riparian areas that lack adequate vegetation or channel/floodplain forms necessary to dissipate energy during flood flows. As a result, they may become unstable during flood events and exhibit excessive erosion or sedimentation, with resulting loss of many of the beneficial functions and values provided by healthy riparian ecosystems. The nonfunctional rating assigned to the Pawnee River riparian area at FOLS is primarily due to a lack of connection between the stream channel and the local water table for any appreciable amount of time during the growing season. This is most likely related to a regional drop in the water table due to decades of groundwater withdrawal. Groundwater levels in the vicinity of FOLS have declined anywhere from about 10 to 20 feet since the mid – 20th century, and while they have apparently stabilized, they are still well below pre-development levels (Balleau Groundwater, Inc., 2006). In addition to the water table decline, the channel has aggraded about 10 feet above its base level due to a concrete check dam just downstream of the park boundary. The sediment that has deposited upstream of the dam since it was constructed exacerbates the problem of disconnection between the channel and the water table. The most dramatic effect of this disconnection is the almost complete lack of perennial riparian-wetland vegetation in the channel bed and on its lower banks. Despite the nonfunctional rating, we don’t believe that the Pawnee River is likely to become destabilized by low to moderate-frequency floods in the near-term because the check dam provides substantial, albeit artificial, grade control. However, if the dam does fail in the future, or it is removed without careful sediment management, the channel bed would incise to obtain a new, stable grade and a great deal of sediment would be discharged downstream. Additionally, the channel banks at FOLS could become de-stabilized if the channel adjusts laterally to the new hydraulic conditions, possibly threatening park infrastructure. Ultimately, such channel adjustments could result in a more functional riparian system, especially if they reestablish a connection between the channel bed/lower banks and the water table. But, the time it would take the river to “re-stabilize” is not predictable, and the degree of potential improvement